Chapter 10: Work and Energy Long Questions | science Class 9 CBSE

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Chapter 10: Work and Energy Long Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. Define 'work' in scientific terms. How does it differ from everyday use of the term?

• In science, work is defined as the product of force and displacement in the direction of the force.
• Mathematically: Work = Force × Displacement (W = F × s).
• Work is done only when displacement occurs in the direction of the applied force.
• If there is no displacement, no work is done even if effort is applied.
• In everyday language, activities like studying or standing with a load are called work, but not in science.
• For example, pushing a wall that doesn’t move is not considered work scientifically.
• Thus, scientific definition emphasizes displacement due to force.

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Question 2

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Q2. What are the essential conditions for work to be done on an object?

• A force must be applied on the object.
• The object must get displaced from its position.
• The displacement should be in the direction of the applied force or have a component along it.
• If either force or displacement is zero, work done is zero.
• No work is done when displacement is perpendicular to the applied force.
• Example: Holding a bag stationary involves force but no displacement, so no work is done.
• Positive work occurs when force and displacement are in the same direction.

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Question 3

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Q3. Explain with examples when work done is positive, negative, or zero.

• Positive work: Force and displacement are in the same direction.
• Example: Lifting an object upwards or pushing a trolley forward.
• Negative work: Force and displacement are in opposite directions.
• Example: Frictional force stopping a moving object.
• Zero work: Either no displacement occurs or the force is perpendicular.
• Example: Carrying a load on your head while walking on a straight path.
• The sign of work depends on the angle between force and displacement vectors.

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Question 4

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Q4. Define 1 joule of work. Derive its SI unit.

• One joule of work is done when a force of 1 newton displaces an object by 1 metre in the direction of the force.
• Mathematically: 1 J = 1 N × 1 m.
• SI unit of force is newton (N), and of distance is metre (m).
• Therefore, unit of work is newton-metre (N·m) or joule (J).
• 1 joule = 1 kg·m²/s² (in base SI units).
• Larger unit: kilojoule (1 kJ = 1000 J).
• Used to quantify mechanical work and energy.

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Question 5

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Q5. What is energy? What are its different forms?

• Energy is the capacity of a body to do work.
• Energy enables objects or living beings to perform tasks like motion, lifting, heating, etc.
• It exists in various forms: kinetic energy, potential energy, heat energy, light energy, chemical energy, electrical energy.
• All forms of energy are interconvertible.
• The SI unit of energy is the joule (J).
• Energy possessed by moving objects is kinetic energy.
• Energy possessed due to position or configuration is potential energy.

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Question 6

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Q6. Define kinetic energy. Derive its mathematical expression.

• Kinetic energy is the energy possessed by a body due to its motion.
• It depends on mass (m) and velocity (v) of the object.
• Work done to accelerate an object from rest to velocity v = kinetic energy.
• Using equation: v² – u² = 2as and F = ma.
• Work done W = F × s = ma × s = ½ mv² (if initial velocity u = 0).
• So, kinetic energy Ek = ½ mv².
• Unit of kinetic energy is joule (J).

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Question 7

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Q7. What is potential energy? Derive the expression for gravitational potential energy.

• Potential energy is the energy possessed by a body due to its position or configuration.
• Example: A stretched spring, water stored at height, compressed rubber.
• Gravitational potential energy is stored when a body is raised against gravity.
• Work done to raise an object = force × height = mgh.
• Therefore, potential energy Ep = mgh.
• m = mass, g = acceleration due to gravity, h = height.
• Unit of potential energy is joule (J).

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Question 8

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Q8. State the law of conservation of energy. Illustrate with the example of free fall.

• Energy can neither be created nor destroyed; it only changes from one form to another.
• Total energy of an isolated system remains constant.
• In free fall, an object at height h has potential energy mgh and kinetic energy zero.
• As it falls, potential energy converts into kinetic energy.
• Just before reaching the ground, kinetic energy is maximum and potential energy is zero.
• Total energy (KE + PE) at all points remains constant.
• This transformation proves the conservation of energy.

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Question 9

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Q9. Define power and average power. What are their SI units?

• Power is the rate of doing work or the rate of energy transfer.
• Formula: Power (P) = Work done (W) / Time (t).
• SI unit of power is watt (W).
• 1 watt = 1 joule/second.
• Larger unit: kilowatt (1 kW = 1000 W).
• Average power = Total work done / Total time taken.
• Power determines how fast work is done.

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Question 10

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Q10. Describe how energy is transferred in the example of a battery lighting a bulb.

• Chemical energy in the battery is converted to electrical energy.
• Electrical energy flows through the circuit to the bulb.
• In the bulb, electrical energy is converted into light and heat energy.
• This transformation allows the bulb to glow.
• Energy flow follows conservation laws—total energy remains the same.
• Batteries are energy sources used in many devices.
• Useful application of energy transformation.

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Chapter 10: Work and Energy | Long Questions

Q1. Define 'work' in scientific terms. How does it differ from everyday use of the term?

Q2. What are the essential conditions for work to be done on an object?

Q3. Explain with examples when work done is positive, negative, or zero.

Q4. Define 1 joule of work. Derive its SI unit.

Q5. What is energy? What are its different forms?

Q6. Define kinetic energy. Derive its mathematical expression.

Q7. What is potential energy? Derive the expression for gravitational potential energy.

Q8. State the law of conservation of energy. Illustrate with the example of free fall.

Q9. Define power and average power. What are their SI units?

Q10. Describe how energy is transferred in the example of a battery lighting a bulb.

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